Interpolating circuit



July 7,l936 H. F. wlLDER 2,046,984

INTERPOLAT ING CIRCUIT F'iled April 5, 1934 .H V1 V. l l ilrold/ Ffmiga? @mam WWW (Ittoltneg Patented July 7, 19.36

UNITED. STATES PATENT orifice INTEEIOLATING CIRCUIT Harold F. wilder,Bloomfield, N. I., signor to The Western Union Telegraph Company, NewYork, N. Y., a corporation of New York Application April 5, 1934,'sox-lai No. 719,193.'-

s claims. (ci. 17a-'10) This invention relates to telegraph systems andparticularly to circuitl arrangements for receiving or repeatingmultiplex code signals wherein the impulses .of the fundamental signalfrequency that have become attenuated in transmission, are automaticallyinterpolated.

It has been disclosed in'recent patents, such as Haglund No. 1,901,296,that the rate` of transmission over cables may be greatly increased byeliminating electro-magnetic relays and employing thermionlc tubes orvalves of the gaseous arc discharge type such as disclosed in reissuepatent to Von Lieben et al., No. 13,779, more fully described inElektrotechnische Zeitschrift of November 27, 1913, at page 1359; andthe Langmuir Patent No. 1,289,823, described in the Journal of theAmerican Institute of Electrical Engineers of November 1928 at page 802,together with other forms in which the characteristics of the Von.

various tube potential functions for the signal f shown in Fig. 2. f

The purpose of my .invention is to provide a.

o circuit arrangement which permits a much the rst harmonic signalfrequency. Combinagreater speed of signaling than has been possibleheretofore, t6 render the reception more sensitive and to automaticallyinterpolate the signal impulses of the highest frequency which have beenattenuated to such a degree that the received energy level for signalsof this frequency is so near theinterference level as to precludefaithful operation of the receiving relay device.

Multiplex code` signals are composed of impulses of a unit time durationof either positive or negative polarity in various combinations and areimpressed upon the line or cable by the telegraph transmitter. In thefollowing description of my invention I shall designate the most rapidalternations of positive and negative potential as tions of two positiveimpulses alternated with two negative impulses will be termed the secondsubharmonic signal frequency. Intermediate to vthese two frequenciesthere lies-a frequency 1.33 times that of the second subharmonic whichis composed of two like impulses followed by a single alternate impulse.Still lower frequency signal combinations will also occur.

Transmission lines absorb energy in an amount logarithmicallyproportional to the frequency of signaling. In the final analysis thespeed of signaling will be limited because of attenuation as thereceiving energyapproaches the energy level 5 of the interference withinthe useful frequency f spectrum. For purposes of discussion, the minimumratio of signal to interference will be said to be 10. VTo increase thespeed of working beyond this limit, certain changes in the localreceiving 10 equipment are essential. This further development isembodied in my presentinvention.

The signaling speed may be increased until the intermediate signalfrequency has suffered sufyiiclent attenuation to bring its Venergylevel to av15 value 10 times that ofthe interference, which is anapproximation of the least ratio found practical in actual operation.The attenuation of the first, harmonic signal frequency will now besuch,

that the received energy level for signals of this 20 frequency will beso near the interference level as to preclude faithful operation of the4receiving relay. If the receiving relay is now connected eithermechanically or electrically to a. rotating machine or commutator, heldin synchronism with the telegraph transmitter, it may be caused tovibratein phase with the first harmonic signal frequency if driven by aneffective force onehalf that of the signal force developed by thereceiving equipment. Signals of a frequency less than the rst harmonicwill hold the relay on its pole or contact against the vibrating force.

o To illustrate this method of multiplex signaling an actual case willbe considered:-

, The line is a non-loaded submarine cable-(Western Union, 1894) rProduct of capacity and resistance- (mi.) :2.425. Interference level0.35 millivolt Maximum signal frequency received and operating the relay11.0 cycles Received millivolts at this frequency for a sending batteryof l0 volts and usual sending capacitive reactance lRatio oi' signal tointerference =10 Equivalent multiplex letters per minute--- =264 Toincrease the speed, the method of signaling previously described andembodying the present invention will be compared below:-`

The signal frequency is increased until the intermediate frequency onlyis operating the relay -.and the attenuated first harmonic isinterpolated by the method herein disclosed. Interference level remainsat. 0.35 m. v. 55

Signal level of nrst harmonic-- (16.5 cycles) 0.38

Signal level of intermediate frequency (l1 cycles) 3.50 m. v.

Equivalent letters per minute 400 Gain in emciency (w- 1oo). =5o% Untilrecently an electromagnetic relay has been used exclusively as theoutput device operated by the signal ampliiied. The considerable powerrequired to properly operate the relay and the ditllculty of maintainingits mechanical functions, which are 'subject to failure and bias fromeither the mechanical or electromagnetic zero,"

can only be appreciated by the engineer or practical designer. At thehigher signaling frequencies a disadvantage is incurred by thenon-linearity of the impedance-frequency characteristic. A gaseousdischarge tube or grid-controlled thermionic rectifier oi the typereferred to, is greatly superior to any magnetic relay inthesecharacteristics. A material reduction in amplification results, asonly one hundredth of a watt at ten volts is required to control a spacecurrent oi several hundred milliamperes in the rectifier tube or valve.The control ratio of the valve or rectier tube is a function oi themechanical spacing oi' the valve elements, the gas pressure and anodepotential, and as these can be readily maintained reasonably constant inoperation, the valve-tube cannot take up an equivalent bias. Making thegrid-cathode external impedance non-inductive will produce a linearfrequency-impedance input characteristic.

The present invention is a circuit arrangement employing gaseousdischarge tubes which are caused to oscillate electrostaticaily insynchronism with the unit signaling frequency and to thereby interpolatethe attenuated ilrst harmonic signal frequency in multiplex signaling.

, Referring to the dramng, the gaseous arc-discharge tube or valve Ti,the resistance R1, the tube or valve Tn and the resistance R: form thefour arms of a Wheatstone bridge circuit. Only one tube is operated atany time so that an alternating potential will be observed across thediagonal of the bridge to which the output, represented by the jack OJis connected. 'I'he grid of each valve is biased 15 volts negativelywith respect to the cathode by the I R drop in resistance Rs of thepotentiometer Ra-i-Rs. At the instant of pick-up, this bias is reducedto onehali.' of the signal voltage.. by shunting Rs with resistance R4synchronously with the signaling frequency, as by means of thesynchronous comgrid pctentiometers Rc-l-Re and Rd+Rf are soproportionally adjusted that if a tube is extingulshed, the maximumpositive grid potential of that tube will rise to one-half the signalvoltage. The time-constant of each circuit is so adjusted that thegradual rise in grid potential reaches a maximum in about 0.9 oi' thetime required for the brush topass from one pick-up segment to abscess.various tube potential functions for the signal shown in Figure 2. Thecycle shown at `l in Fig.

2, illustrates signal impulses transmitted at the rate of the rstharmonic signal frequency. As

indicated in Fig. 3, the attenuation in transmission through the cableis so great that the amplitude of the received signals oi first harmonicirequency is insuflicient to actuate a relay. The graph `of the signalE. M; F. shown at E in Fig. 3, is rst positive for tube T1 and, ofcourse, is negative for tube Tn as the grids of the tubes are connectedin opposition across the output resistance In this description I shallconsider only the potential on the grid of tube T1..

In Figure 4 the negative bias voltage set up across resistance R3 isshown to be periodically decreased from -15 to -5 volts during the timethat the pick-up brush b is in contact with a pickup segment. The uppercurve or Figure 4 represents the algebraic sum of the signal voltage Eand the E. M. F. fed back to the grid by the potentiometer and delaycircuit. This E. M. F. is indicated by the dotted line e1 of Fig. 3 andrises to a maximum of ilve volts positive when the tube T1isout and T2is on. Similarly, the feed back E. M. F. es acts on the grid of tube Tabut electrical degrees out of phase with e1. Ii the delay circuits wereomitted these E. M. FXS would reach a maximum during the pick-up timevandboth tubes would come on", thereby blocking the signals.

lt will be convenient to refer to Figure 6 to observe the time duringwhich tube T1 is "on". and HOE".

Only the actual potential diilerence between the grid and cathode of thetube is eiectlve in its control and to show this controlling voltageboth -plots of Fig. 4 have been added algebraically as shown in Fig. 5.At the instant of pick-up numbered i, 2 and 3, a positive signal oi' 10volts is opposed by the negative 5-vo1t bias, momentarily reduced from15 volts. 'I he resultant positive ilve volt E. M. F. acting on the gridstarts the'tube T1. As a matter of fact, the grid of the tube cannotrise much above 2 volts positive because it is in contact with the arcdischarge, but-for the present discussion this may be neglected as thegrid E. M. F. is not so affected during the out" period preceding thedischarge. At the instants i, 2, and I, it will be observed that thefeedback E. M. F. en on tube T: is positive at the moment of pickup, butis more than overcome by the 10 volt negative signal. At the instant 4,however, the signal has dropped to zero and the tube T: starts.

The reverse is true at 5 where er is now a maximum and tube T1 isoperated, extinguishing tube T2. In this manner the tubes oscillateduring the interval that the signarlevel is. zero.

A negative signal at the instant i, prevents the Vtube T1 from beingoperated, for the etlective grid bias is maintained highly negative atall times. The rise in a positive direction finally reduces theeffective grid potential and the tube T1 starts at instant 1.

I have described the circuit arrangement shown in the accompanyingdrawing in detail for the' puIDOse of clearly disclosing one embodimentoi my invention but it will be evident to 75 being provided for eachpolarity of current, a,

. s,o4e,e's4 engineers that various changes and modifications adischarge therethrough in synchronism with can be made within the scopeof my claims.

1. In a signaling system, means for producing groups of uniform numbersof selecting signal conditions, a line circuit including a gaseousconduction path, biasing means normally preventing a discharge acrosssaid path, means for periodically changing said biasing means insubstantial synchronism with said selecting conditions to permit adischarge across said path and means i'or j interrupting said dischargeselectively in accordance with, the polarity of the successive signalconditions.

2. In a signaling system, a source of periodic signals of positive andnegative polarity, receiv-` ing apparatus for said signals comprising agasecus conduction device for each polarity of signaling current, eachdevice having anode, cathode and grid elements, means for normallyapplying a predetermined bias on said grids, means for periodicallyreducing the value ci said bias to permit a discharge through one or theother of said devices in accordance with the signal polarity and meansassociated with said ydevices for causing the discharge through one tubeto interrupt the discharge through the other tube.

3. In a signaling system, a source of periodic signals of oppositepolarity, responsive apparatus for said signals comprising gaseousdischarge tubes having control grids for initiating discharges throughthe tubes, one of such tubes common biasing means for the tubes, meansoperating in synchronism with the signals for periodically. shuntingsaid biasing means to permit a discharge through one tube in accordancewith the polarity of the signal and circuit connections -for causing thestarting ot one tube to interrupt the operation of the other tube.

4. I n a signaling system, a source of periodic signals of oppositepolarity, responsive apparatus for said signals comprising gaseousdischarge tubes having control grids for initiating discharges `throughthe tubes, one ci.' such tubes being provided for each polarity of'current, control circuits for said tubes associated with said grids for4starting a discharge therethrough in accordance with the polarity of.said periodic signals, circuit connections for causing the interruptionot one tube upon the starting of the other tube, and storing meansconnected to each grid, operating to impress an electromotive forcethereon oi.' the properV phase and polarity to initiate a dischargethrough the interrupted tube in synchronism with the next succeedingsignal.

5. In a signaling system, a source o! periodic signals of oppositepolarity, responsive apparatus for said signals comprising gaseousdischarge tubes having control grids i'or initiating discharges throughthe tubes, one oi' such tubes being provided for each polarity ofcurrent, control circuits for said tubes associated with said grids forstarting a discharge therethrough in accordance with the polarity oisaid periodic signais, circuit connections for causing the interruptionof one tube upon the startingof. the

other tube. and means for impressing an elec-` tromotive force upon thegrid of the interrupted tube of the proper phase and polarity toinitiate starting of a` discharge therethrough in'accordbut irrespectiveof the next succeeding signal. 6. In a signaling system, a line circuitconnected to a source of periodic telegraph signals of marking andspacing conditions, apparatus 5 with. the signal condition.

7. In a signaling system, a line circuit a source of periodic signals ofpositive and negative polarity selecting conditions operating at a speedoi transmission such that the signal impulses of fundamental frequencyor unit time duration are attenuated below the effectiveoperating level,means for interpolating said attenuated impulses, comprising anelectrostatically controlled arc discharge tube for each polarityconnected to the line, each tube having a grid controlling the 25 ancewith each successive selecting' condition, circuit arrangements forextinguishing one tube upon the starting of the other tube, and meansfor impressing an electrostatic charge upon the grid of the interruptedtube of the proper polarity to initiate a discharge therethrough whenthe next succeeding impulse is of unit time duration.

y 8. A signaling system as defined in claim 7, said means for impressingan electrostatic charge 35 upon the grid oi' the interrupted tubeoperating .upon each tube in succession to cause the tubes to oscillatein synchronism with the transmitted signals when the signal level fallsbelow the effective operating level and thereby to interpo- 40 late thesignal impulses of unit time duration.

9. In a signaling system, a line or cable, a source of periodic signalsof positive and negative polarity operating at a speed of transmissionsuch that the signal impulses of fundamental 45 frequency or unit timeduration are attenuated below the effective operating level, a secondline or receiver, and means to repeat said signals from said first lineor cable into said second line or receiver; comprising gaseous discharge50 tubes havingV control grids for initiating discharges through thetubes, a tube being provided for each polarity, the input' of said tubesbeing continuously connected to said iirst line or cable and the outputoi' said tubes being con- 55 tinuously connected to said second line orreminou: F. wxmm. 7o

